Signal transmission system



27, 1 J. H. BOLLMAN SIGNAL TRANSMISSION SYSTEM Filed Feb. 11} 19:59

4 Sheets- Sheet l INVENTOR By J.h! BOLLMAN Wj e /w ATTORNEY J. H. BOLLMAN S IGNAL TRANSMISS ION SYSTEM Aug. 27, 1940.

Filed Feb. 11, 1939 4 Sheets-Sheet 2 lNl ENTOR JHBOLLMAN ATTORNEY Aug. 27, 1940. J. H. BOLLMAN 2,213,037

\ SIGNAL TRANSMISSION SYSTEM Filed Feb. '11, 1939 4 Sheets-Sheet 5 l n I mi F- k5 lg.

in m VFW mm W m N Q G 0 A 4, '0 t 2 M JAIL JLQJL mi; m m" W ZZZ 4% A T TOR/V5 V 27, 1 J.- H. BOLLMAN SIGNAL TRANSMISSION SYSTEM Filed Feb. 11, 1939 4 Sheets-Sheet 4 N on 5a mmmuwm WWW lA/VENTOR J./-/. BOLLMA/V Wm M ATTORNEV Patented Aug. 27, 1940 UNITED STATES PATENT OFFICE Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application February 11, maserialNo.255,9s2

10 Claims.

"This invention relates to signal transmission systems and particularly to pilot channels for carrier current systems.

One object of the invention is to provide a sig- IY5. nal transmission system having amplifiers located at a main station and at auxiliary stations with a source of current at the main station controlled by a pilot current to govern the amplifiers at said stations.

Another object of the invention is to provide a signal transmission system having a pilot channel-and carrier channels connected to amplifiers located at a. plurality of stations that shall control a low frequency current at one station according to the pilotcurrent on the pilot channel to govern the amplifiers at said stations. Another object of the invention is to provide a cable systemhaving carrier channels on the difierent pairs of conductors and a pilot chan- 20 nel on one of the pairs of conductors with amplifiers located at main and auxiliary stations that shall govern a control current at a main station according to the pilot current on the pilot channel to govern the amplifiers at each of said stations.

"A further object of the invention is to provide a signal transmission system having amplifiers governed by heat controlled resistance elements and parallelly arranged heater coils controlling the resistance elements that shall have a variable control current energizing said heater coils and an auxiliary constant source that shall operate said heater coils to heat the resistance elements'at 'a'fixed temperature in case of "failure of said control current.

In along signaltransmission system it is necessary to provide repeaters or amplifiers at intervals along atransmission line to maintain the 'levelof'the transmitted signals. The amplifiers along the transmission line should be controlled according to the losses efiected by the signals to holdthe signal at 'a fixed level. A carrier cable "system may be provided with a number of carrier channels on each pair of conductors and. With an amplifier'for'each pair of conductors at each mainor-auxiliary station. It is desirable to provide one or more pilot channels on the cable pairs of conductors to control the amplifiers at :the Various stations.

Thepresent invention has been illustrated by *meansrof .1a cablecarrier system. A cable containing a number of pairs of conductors is pro- .vided with amain station and a number of auxiliary stations. At each station an amplifier is provided. for the channels on each pair of conductors. ,Eachpair of conductors has a number of'carr-ier channelsand one pair of conductors hasapilot channel thereon. A control current .is provided at the main station for governing .the amplifiers at .the main station and all the auxiliary stations. The control current from the source at the main station is controlled in accordance with the amplitude of the pilot current on the pilot channel at the main station.

Inone form of the invention the amplifiers at each stationare provided with feedback circuits controlled'by resistance elements having a high negative temperature coeflicient of resistance such, for example, as silver sulphide. It is obvious thatelements having high positive temperature coeificients of resistance may be employed. Heater coils are provided for heating the resistance elements at each station. The heatercoils at'each stationare connected in parallel tothesource of control current at the main station. The control current supplied to the amplifiers at eachstation is adjusted to control the rate of gain change at each station and to control the flat gain-at each station. The control current at the *main station which is controlled in accordance with the pilot current on the pilot channel may not only be supplied to auxiliarystations butalso to another main station. The control current which is supplied to a secondmain station may be amplified and then employed to control another set-of auxiliary stations. -'T-he rate of gain change and the fiat gain at each station under controlof the second attendedstationmay be adjusted as above set forth.

Provision may be made to prevent abnormal gain changes in the signal transmission in case -offailure of the control current supplied to the amplifiers at any station. A cold cathode gasfilled tube having an ignition circuit and an operating circuit may be employed for insuring a fixed gain bythe amplifiers at each station in caseof 'failureof the controlcurrent. The control current supplied to the heater coils at a station is connected to the ignition circuit of said tube in opposition to 'a fixed potential source. Thefixed potential source is of substantially the same strength as the control current to prevent ignition of the tube under normal conditions. The fixedsource of potential is also connected t the operating circuitof the tube in circuit with the heater coils at-the station. In case of failure or the control current the tube is ignited and the heater coils are operated by power supplied to the operating circuit of the tube by the fixed source of potential. When control current is againreceived the tube isextinguished and power .is then supplied to the heater coils by the source .of control current.

.In a modification of the invention main and auxiliary. stations on a coaxial cable system may be controlled as above set forth. The coaxial ca- .ble will not only have carrier channels but also a pilot channel. The pilot current on the pilot channel will-be filtered from the carrier currents .at .a .mainstationanda control current at the main station will be governed according to the amplitude of the filtered pilot current. The control current governed by the pilot current will be employed to govern the amplifiers at the main and auxiliary stations. The control current supplied to the amplifiers at the various stations may be adjusted, if so desired, to control the rate of gain change and the fiat gain at the various stations.

In the accompanying drawings, Figs. 1, 2 and 3 are diagrammatic views of a transmission systern constructed in accordance with the invention. The drawings are assembled to show the complete system by attaching Fig. 2 to the left of Fig. 1 and by attaching Fig. 3 to the left side of Fig. 2; and

Fig. 4 is a diagrammatic view of a modification of the system shown in Figs. 1, 2 and 3 of the drawings.

Referring to Figs. 1, 2 and 3, a cable system is illustrated wherein carrier currents are transmitted by different pairs of conductors in a cable. In the drawings, only four pairs of conductors, numbered 2, 3 and 4, are illustrated. It is to be understood that the cable may be provided with any desired number of pairs of conductors which are controlled in the same manner as the pairs of conductors 2, 3 and 4.

A main station 5 and three auxiliary stations 6, and 8 are located along the cable. At each station an amplifier is provided for each pair of conductors. The amplifiers at each station are controlled from the main station 5 according to the amplitude of the pilot current on a pilot channel of one of the pairs of conductors. At the main station 5 amplifiers 9, II], II and i2 are respectively provided for the pairs of conductors I, 2, 3 and 4. At the auxiliary station 6 amplifiers l3, l4, l5 and I6 are respectively provided for the pairs of conductors 2, 3 and 4. At the auxiliary station 1 amplifiers I1, l8, l9 and 20' are respectively provided for the pairs of conductors I, 2, 3 and 4. At the auxiliary station 8 amplifiers 2|, 22, 23 and 24 are respectively provided for the pairs of conductors 2, 3 and 4.

The amplifiers at each of the stations are of the feedback type and are controlled by varying a resistance element in the beta circuit. Preferably, the resistance element in the beta circuit of each amplifier has a high negative temperature coefficient of resistance and may, for example, be boron or silver sulphide. The amplifiers 9, III, II and I2 at the main station 5 have resistance elements 25, 26, 21 and 28 located in the beta circuits. Heater coils 29, 30, 3| and 32 are respectively provided for heating the resistance elements 25 to 28, inclusive. The beta circuits of the amplifiers 3 to |6, inclusive, located at the auxiliary station 6, are respectively provided with resistance elements 33 to 36, inclusive. The resistance elements 33 to 36, inclusive, are respectively provided with heater coils 31 to 40, inclusive. The beta circuits of the amplifiers I! to 26, inclusive, at the auxiliary station 1 are controlled by resistance elements 4| to 44, inclusive, The resistance elements 4| to 44, inclusive, are respectively provided with heater coils 45 to 48, inclusive. The beta circuits of the amplifiers 2| to 24, inclusive, at the auxiliary station 8 are respectively provided with resistance elements 49 to 52, inclusive. The resistance elements 49 to 52, inclusive, are respectively provided with heater coils 53 to 56, inclusive.

The pairs of conductors I, 2, 3 and 4 may not only have carrier channels but may have pilot channels thereon. The pair of conductors is assumed to have a pilot channel which carries pilot current from a pilot generator 51. At the main station 5 the pilot current from the generator 51 is selected from the carrier currents by means of a pilot filter 58. The pilot filter 58 is connected to an amplifier tube 59 by means of a transformer 60. The amplifier tube 59 is connected by a transformer 6| to a copper-oxide rectifier 62 of the doubler type. The rectifier 62 is connected to a control tube 63, which in turn is connected by a transformer 64 to an amplifier tube 65. Battery |4fl placed between the rectifier 62 and the tube 63 opposes the rectified voltage in order to obtain voltage changes which are more than proportional to the changes by the pilot current. The amplifier tube 65 is connected by a transformer 66 to an adjustable resistance pad 67. The resistance pad 67 is adjusted to control the rate of gain change by the amplifiers 9 to |2, inclusive, at the main station 5. A second resistance pad 68 serves to control the flat gain of the amplifiers 9 to |2, inclusive, at the main station 5. The resistance pads 6'! and 68 are connected to each other and to a control circuit '69 by means of a hybrid coil 10.

A source of low frequency current 1| is provided at the main station 5 and may be a (SO-cycle power circuit. The source 1| is connected by a transformer 12 to the plate circuit of the control tube 63. The low frequency current supplied to the plate circuit of the tube 63 is controlled according to the amplitude of the pilot current on the pilot channel at the main station 5. Low frequency current from the source H is also supplied to the resistance pad 68.

A tube 73, which may be a gas-filled tube or a cold cathode tube is provided for insuring the heating of the coils 29 to 32, inclusive, for the resistance elements 25 to 28, inclusive, at a fixed temperature in case of failure of the control current supply by the control circuit 69. The control circuit 69 is connected not only in parallel to the heater coils 29 to 32, inclusive, but also to a coil 14. The coil 14 is inductively connected to a coil 15 in the ignition circuit of the tube 73. The tube 13 is provided with three electrodes l6, "l1, and 18. The electrodes 76 and T! are in the ignition circuit of the tube and the electrodes 18 and l! are included in the operating circuit of the tube 73. The ignition circuit of the tube 73 not only includes the electrodes l6 and H, the coil 15 and a resistance element 19, but also the secondary winding 80 of a transformer 8|. The primary winding of the transformer 8| is connected to the source of low frequency current 7|. The operating circuit of the tube l3 includes the electrodes 11 and 18, secondary Winding 80 of the transformer 8|, coil 14 and an adjustable resistance 82.

In a circuit constructed as above set forth the gain of the amplifiers 9 to |2, inclusive, at the main station 5 is controlled according to the amplitude of the pilot current on the pilot channel at the main station 5. The pilot current is selected from the carrier currents at station 5 by means of the filter 58. The pilot current is then amplified by the tube 59 and rectified by the rectifier 62. The rectified pilot current governs the operation of the control tube 63. Low frequency current from the source 1| is supplied to the control tube 63 by means of the transformer 12. The low frequency current is controlled according to the pilot current by means of the control tube 63. The low frequency current governed according to the pilot current is amplified by the tube and supplied to the gain rate adjusting pad 6?. The pad 6?, as before set forth, is adjusted to control the rate of gain by the amplifiers at the station 5 and the pad 68 is adjusted to adjust the flat gain by the amplifiers at the main station 5. The fiat gain pad '68 is connected to the source El and the two pads 67 and 68 are connected by the hybrid coil l0 to the control circuit The control circuit 69 is connected in parallel to the heater coils 29 to 32, inclusive. The heater coils 29 to 32, inclusive, control the resistance elements 25 to 28, inclusive, which in turn govern the amplifiers 9 to I2, inclusive.

The low frequency control current from the circuit 69 energizes the coil M, which is inductively connected to the coil 15 in the ignition circuit of the cold cathode tube 13. The power supplied to the ignition circuit from the coil I4 is opposed by power supplied from the low frequency source 'II by means of the transformer 8 I. The power from the two sources oppose each other and prevent ignition of the tube I3. The two sources of power supplied to the ignition circuit of the tube I3 have the same frequency and are in phase opposition. In case of failure of the control circuit 69, the coil I5 in the ignition circuit of the tube I3 is deenergized and immediately the tube 73 is ignited by power supplied from the source H Upon ignition of the tube I3 power is thensupplied to the heater coils 29 to 32, inclusive, from the source 'II through the transformer 8I. The heating of the resistance elements 25 to 28, inclusive, by power supplied from the source Ilthrough the transformer 8! is presumed to hold the amplifiers 9 to I2, inclusive, at an average gain.

The controlled low frequency current from the amplifier .65 is not only supplied to the rate adjusting pad 61, but also to a rate adjusting pad 83 for the auxiliary station 6 and to an amplifier 86 for controlling the amplifiers at the main station 8 and the amplifiers at-the auxiliary station "I. The pad 83 controls the rate of gain by the amplifiers I3 :to I6, inclusive, at the-auxiliary station 6. An H pad 38 is provided tocontrol the fiat gain by the amplifiers E3 to I6, inclusive. The pad.83 is connected to the transformer .66 as before set forth and the pad 88 is connected to the source of low frequency current II. The two pads 83 and -88 are connected together and to control circuit by means of a hybrid coil 89. The rate of gain pad is employed to adjust the amplifiers for the different conditions .to which the cable sections are subjected. One section of the cable controlled by the amplifiers of one station may be underground whereas another section of the cable controlled by the amplifiers at another station may be above ground. The rate of gain change for controlling the underground section must be different from the rate of gain change controlling the section of cable located above ground. The flat gain pad is employed to adjust the amplifiers for the length of cable controlled by them.

The control circuit 85 is connected by a transformer 89A to a phantom circuit formed by the pairs of conductors I and 2. A transformer 66 at the auxiliary station 6 is connected to the phantom circuit formed by the pairs of conductors I and 2. The secondary winding of the transformer 98 is connected in parallel to the heater coils 37 to 40, inclusive, which control the resistance elements 33 to- 36, inclusive. The resistance elements 33 to 36, inclusive, at the auxiliary station 6 are controlled in the same manner as the resistance elements 251 to 28, inclusive, at

station 5 are controlled.

A three-element tube 9| .at station 6 is similar in construction and operation to .the three-element tube 13 at the main station 5. The power circuit 87 which connects the transformer 8| at station 5 to the source of low frequency cur-rent 'II also connects a transformer 92 at station-6 .to the source of low frequency current. The circuit 8! maybe one or more pairs of conductors in the cable or may be a phantom circuit formed by two pairs ,of conductors in the cable. The secondary winding of the transformer 92 is connected to the ignition circuit and the operating circuit of the tube M. The tube 9i operates in the same manner as the tube I3 for operating the heater elements v3'I to 46, inclusive, in case of failure of the power supplied by the control circuit 85.

The control circuit 86 at the main station 5 is connected by a transformer 93 to a phantom cir-, cuit formed by the pairs of conductors 3 and 4 between the stations 5 and 6. A transformer 94 at station 6 is connected to the phantom circuit formed by the pairs of conductors 3 and 4 between the stations 5 and 6. The secondary winding of the transformer 94 is connected to the primary winding of a transformer 35. The secondary winding of a transformer 95 is connected to a phantom circuit formed by the pairs of conductors 3 and 4 between stations '6 and "I. A transformer 36 at station I has the primary winding connected to the phantom circuit formed by the pairs of conductors 3 and 4 between the stations 6 and I. The secondary winding of the transformer 96 is connected to the primary winding of a transformer 91. The secondary winding of the transformer S? is connected to a phantom circuit formed by the pairs of conductors 3 and 4 between stations I and 8. At station 8 a transformer 97A has the primary winding connected to the phantom circuit formed by the pairs of conductors 3 and 4 between thestations I and 8.

The secondary winding of the transformer 91A is l connected by a transformer 98 to the amplifier $4. The amplifier 84 is connected by a transformer 98 to H rate of gain pads I06 and HM. The pad I66 controls the rate of gain effected by the amplifiers H to 24, inclusive, at station 8.

The pad I8I controls the rate of gain effected by the transformers I! to 28, inclusive, at the auxiliary station "5. A flat gain pad I62 is connected to the rate of gain pad I06 by means of a hybrid coil I63. A fiat gain pad I64 is connected to the rate of gain pad IllI by means of a hybrid coil I65. The flat gain pads I62 and I04 are connected to the source of low frequency current "II by means of the circuit 81.

The heater coils 53 to 56, inclusive, at station 8 connected to the pads Hi8 and I62 by means of the hybrid coil I83. The heater coils 53 to 55, inclusive, are controlled according to the pilot current station 5 and according to the adjustment of the rate of gain pad and fiat gain pad to P control the amplifiers 2I to 24, inclusive. The operation of the amplifiers 2| to 2d, inclusive, at station 6 is similar to the operation of the amplifiers 6 to I2, inclusive, at station 5. A three-element tube I66, which is similar in construction and operation to the tube I3 at station 5, is employed for controlling the gain of the amplifiers M to 24, inclusive, in case'of failure of the power under control of the pilot current on the pair of conductors I. The circuit 81, which is connected 7 5 Til) to the source of low frequency current II, is connected by a transformer I07 to the ignition circuit and the operating circuit of the tube I 06. The power supplied by the transformer IB'I opposes the power supplied to the ignition circuit of the tube I 06 from the control circuit connected to the heater coils 53 to inclusive. Upon failure of the power supplied to the heater coils, the ignition circuit of the tube I66 breaks down the tube to effect operation of the operating circuit. The operating circuit of the tube I supplies power from transformer NJ! to the heater coils 53 to 56, inclusive, to hold the gain of the amplifiers 2I to 24, inclusive, at some normal amount.

The hybrid coil I85, which is connected to the pads IBI and IE4, is also connected to the primary winding of a transformer IE8. The secondary winding of the transformer I88 is connected to a phantom circuit extending between the station 3 and station I on me pairs or conductors I and 2. A transformer I09 at station I has the primary winding connected to the phantom circuit extending between the stations I and 8 on the pairs of conductors I and 2. The secondary winding of the transformer I09 is connected in parallel to the heatercoils to 48, inclusive. The heater coils 45 to 48, inclusive, control the resistance elements 4! to M, inclusive, to control the gain effected by the amplifiers I? to 29, inclusive. In the circuit above traced the amplifiers I! to 20, inclusive, are controlled according to the amplitude of the pilot current at station 5. The rate of gain by the amplifiers at station I is controlled by adjusting the pad Illl and the fiat gain by the amplifiers at station I is controlled by adjusting the pad I85. In the control of the amplifiers at station "I it will be noted that a low frequency current at station 5 governed according to the pilot current is transmitted to station 8 and from station 8 to station I.

A three-element tube III], which is similar in construction and operation to the tube 13 at sta tions 5 is employed for effecting heating of the coils 45 to 18, inclusive, in case of failure of the normal operating power supply to the coils. A transformer I! I is provided for connecting the ignition circuit of the tube IIII to the circuit 8" connected to the source of low frequency power II at station 5. The tube III! is operated in the same manner as the tube I3 at station 5.

In the circuit above described it will be noted that only one band-pass filter is required for controlling the amplifiers at a number of stations. The pilot current is selected at only one station, namely station 5. The pilot current which is selected at station 5 is employed for controlling a low frequency current. The low frequency current is transmitted to various other stations for control purposes.

Referring to Fig. 4 of the drawings, a modified control circuit is shown for use on a coaxial cable system. Three stations comprising a main station H2 and two auxiliary stations H3 and H4 are shown connected to the coaxial cable comprising conductors H5 and IE8. Three amplifiers II'I, IE8 and H9 are respectively provided at the three stations H2, I I3 and HQ. The amplifiers III to III], inclusive, are of the feedback type having the beta circuits thereof controlled by a resistance element having a high negative temperature coefficient of resistance. Resistance elements I to I22, inclusive, respectively associated with the beta circuits of the amplifiers II! to H9, inclusive, are assumed to be composed of silver sulphide. Heater coils I23 to I25, inclusive, are provided for heating the resistance elements I20 to I22, inclusive.

A pilot current control circuit I26 is connected to the coaxial cable at the main station II2 for controlling the amplifiers II! to H9, inclusive. The pilot current control circuit I26 is similar to the pilot current control circuit shown in Fig. 1 of the drawings. This control circuit may comprise a pilot filter, an amplifier, a rectifier, a control tube and an amplifier tube similar to the parts 58, 59, 62, B5 and 65 shown in Fig. 1 of the drawings. The circuit I26 is connected to a circuit l2! extending between the stations H2, I I3 and H4. The circuit I2! is connected to the heater coil I23 by means of a transformer I28. The circuit I2! is connected to the heater coil I24 by a transformer I29. The heater coil I25 is connected to the circuit I22. by a transformer I30. L pads I3I, I32 and I33 are respectively placed in the connections to the coils I23, I24 and I25 for controlling not only the rate of gain but also the fiat gain by the amplifiers at stations H2, H3 and Hi. It is assumed in the circuit under consideration that the sections of the cable controlled by the amplifiers H1, H8 and H9 are all subjected to substantially the same temperature conditions.

In the circuit shown in Fig. 4 in the drawings the pilot current on the coaxial cable is selected and employed for controlling a low frequency current at the station I I2. This low frequency current is then transmitted by the circuit I21 to the various stations for controlling the gain at each of the stations. The pilot current control circuit operates in the same manner as the pilot current control circuit described when reference is made to Figs, 1, 2 and 3 of the drawings.

Modifications in the circuits and in the arrangement and location of parts may be made within the spirit and scope of the invention and such modifications are intended to be covered by the appended claims.

What is claimed is:

1. In combination, a transmission line having a signal channel and a pilot channel, a number of stations on said line, an amplifier at each of said stations for the signal and pilot channels, a source of low frequency current at one of said stations, and means for controlling the amplitude of said low frequency current according to the amplitude of the pilot current on the pilot channel, for feeding said varied low frequency current back to the various stations and for controlling the amplifier at each station according to the amplitude of the varied low frequency current.

2. In combination a pair of conductors having a number of carrier channels and a pilot channel thereon, a main station and auxiliary stations connected to said conductors, each of said stations having an amplifier for the carrier and pilot channels, a source of alternating current at the main station, means free from moving parts at said main station for varying the amplitude of said alternating current according to the strength of the pilot current at the main station, and means for controlling the amplifiers at said stations according to the amplitude of the alternating current.

3. In a carrier current transmission system, a transmission line having a number of carrier channels thereon, a pilot channel on said line transmitting a pilot current of relatively high frequency, main and auxiliary stations on said line, each of said stations having an amplifier for the'carrier and pilot channels, a source of relatively low frequency current at the main station, means free from moving parts at said main station for filtering said pilot current from the line and for varying the amplitude of said low frequency current under control of said pilot current, and means controlled according to the amplitude of said low frequency current at each of said stations for governing the amplifiers at said stations to control the gain on said line.

4. A transmission line having a signal channel and a pilot channel thereon, a main station and a number of auxiliary stations on said line, anamplifier at each of said stations, a source of low frequency current at the main station, means at said main station for selecting the pilot current on the pilot channel from the signal currents and for rectifying the selected pilot current, means comprising a control tube governed by said rectified pilot current at the main station for modulating said source of low frequency current to produce a low frequency control current having an amplitude varying according to variation in the selected pilot current at the main station, and means for transmitting said control current to each of said stations and for controlling the amplifier at each of said stations according to variations in the control current.

5. A cable having a number of pairs of conductors thereon, carrier channels on each of said pairs of conductors, a pilot channel on one of said pairs of conductors, a main station and a plurality of auxiliary stations on said cable, an amplifier for each of said pairs of conductors at each of said stations, a source of low frequency current at the main station means at said main station for selecting the pilot current from the carrier currents on the pair including the pilot channel and for rectifying the selected pilot current, means comprising a control tube governed by said rectified pilot current at the main station for modulating said source of low frequency current to produce a relatively low frequency current having an amplitude varying according to variations in the pilot current, and means for transmitting said low frequency current to each of said stations and for controlling the amplifiers at each of said stations according to variations in the low frequency current.

6. In combination, a cable having a number of pairs of conductors therein, carrier channels on each of said pairs of conductors, a pilot channel on one of said pairs of conductors, a gain control station on said cable, an amplifier having a beta circuit for each of said pairs of conductors at the gain control station, resistance elements having high negative temperature coefficients of resistance for controlling the beta circuits of said amplifiers, a plurality of parallelly connected coils for heating said resistance elements, a source of low frequency control current connected to said heater coils for controlling said resistance elements, means for controlling the amplitude of the low frequency current according to the amplitude of the pilot current at said station, a three-element cold cathode tube having the starting electrodes connected to said source of control current in parallel to said heater coils and a source of constant potential connected to said starting electrodes in opposition to said control current to prevent ignition of said tube and connected to the third electrode of said tube in circuit with said heater coils to effect constant heating of said coils in case of failure of said control current and ignition of said tube.

7. In combination, a transmission line having a number of carrier channels and a pilot channel, a main station and auxiliary stations on said line, an amplifier at each of said stations for the carrier and pilot channels, a source of low frequency current at the main station, means for controlling the amplitude of said low frequency current according to the amplitude of the pilot current on the pilot channel at the main station, for separately supplying said varied low frequency current to the various stations and for controlling the amplifier at each station according to the amplitude of the varied low frequency current, and means for varying the low frequency currents respectively supplied to the stations to control the rate of gain change and the fiat gain change effected by the low frequency control current at the various stations.

8. In combination, a cable having a number of pairs of conductors therein, carrier channels on each of said pairs of conductors, a pilot channel on one of said pairs of conductors, a main station and a number of auxiliary stations on said cable, an amplifier for each of said pairs of conductors at each of said stations, a source of low frequency current at said main station, means for controlling the amplitude of said low frequency current according to the amplitude of the pilot current at the main station, for separatelyv supplying said varied low frequency current to the various stations and for controlling the amplifiers at each station according to the varied low frequency current, and means for varying the low frequency currents respectively supplied to the stations to control the rate of gain change and the fiat gain change effected by the low frequency current at the various stations.

9. In combination, a transmission line having a number of carrier channels and a pilot channel, a number of stations on said line, an amplifier for the carrier and pilot channels at each of said stations, a source of current at one of said stations, means for controlling the amplitude of the current from said source according to the amplitude of the pilot current on the pilot channel, means for feeding said controlled current to the various stations, means for adjusting the controlled current according to the flat gain and the rate of gain change desired for change in the pilot current, and means for controlling the amplifier at each station according to the amplitude of the controlled current from said source.

10. In combination, a transmission line having a number of carrier channels and a pilot channel, a number of stations on said line, an amplifier for the carrier and pilot channels at each of said stations, means comprising an element having a large temperature coeificient of resist ance associated with each of said amplifiers for controlling the gain, a source of current at one of said stations, means for controlling the amplitude of the current from said source according to the amplitude of the pilot current on the pilot channel, means for feeding said controlled current to the various stations, means for adjusting the controlled current according to the flat gain and rate of gain change desired for change in the pilot current and means for controlling the resistance elements respectively associated with the amplifiers according to the amplitude of the controlled current from said source.

JOHN H. BOLLMAN. 

